In the field of construction of dwellings, commercial buildings, and similar structures, there has been a great increase in recent years in the use of windows and vents which employ sliding panels. These windows and vents usually comprise a pair of parallel, spaced apart, horizontal channel members, each having a pair of adjacent tracks therein which receive the window or vent panel and permit them to be slidably and horizontally translated. Often these units are preassembled by original equipment manufacturers using end rails or channel members to join the horizontal channel members together with the panel assembled therein. The prefabricated units are then installed with a minimum of labor in wall openings which are dimensioned to receive the units. Although sliding panel vents and window units cannot be blown open by the wind, they are quite susceptible to being opened in an unconventional manner by vandals and theives. To remedy this situation, there have been devised in the prior art many forms of locks to seal the units and prevent unauthorized entry therethrough. Other lock devices limit the opening afforded by the vent or window to prevent entry therein. In general, these lock devices are an added expense for each window, an expense which is greatly multiplied in a structure having many windows. Also, many of the lock units which require the proper keys for opening are a safety hazard in an emergency, in that they do not permit egress without the use of the proper key. During fires or similar emergencies, the time wasted unlocking a key-locked sliding window may determine the difference between escape and injury or death. Also known in the prior art are devices which obstruct the track of the channel members to limit the opening of the sliding panel. These devices often screw or bolt to the channel member of thr unit, and are thus poorly adapted to provide variable opening of the unit. These devices also add to the cost of each window or vent unit. Furthermore, these devices are removable from the channel member, and must be reinstalled before each use. Moreover, it has been found that these removable devices tend to be pilfered or otherwise permanently removed from window units in hotels, motels, and similar dwellings.